The next generation of solid-state lighting is seeking to provide advances in brightness, efficiency, color, purity, packaging, scalability, reliability and reduced costs. The creation of light emitting devices from silicon based materials, upon which the modern electronic industry is built, has been the subject of intensive research and development around the world. The main obstacle has been the indirect energy gap of bulk silicon, which limits the efficiency to an extremely low level. However, one particular technology, based on silicon nano-particles, e.g. nanocrystals, formed through various techniques, has been able to overcome this difficulty.
Prior art light emitting devices, such as those disclosed in U.S. Pat. Nos. 7,081,664, entitled: “Doped Semiconductor Powder and Preparation Thereof”, issued Jul. 25, 2006 in the name of Hill; and 7,122,842, entitled Solid State White Light Emitter and Display Using Same, issued Oct. 17, 2006 to Hill; and United States Published Patent Applications Nos. 2004/151461, entitled: “Broadband Optical Pump Source for Optical Amplifiers, Planar Optical Amplifiers, Planar Optical Circuits and Planar Optical Lasers Fabricated Using Group IV Semiconductor Nanocrystals”, published Aug. 5, 2004 in the name of Hill; 2004/214,362, entitled: “Doped Semiconductor Nanocrystal Layers and Preparation Thereof”, published Oct. 28, 2004 in the name of Hill et al; and 2004/252,738, entitled: “Light Emitting Diodes and Planar Optical Lasers Using IV Semiconductor Nanocrystals”, published Dec. 16, 2004 in the name of Hill, which are incorporated herein by reference, have demonstrated that using silicon-rich silicon oxide (SRSO), which consists of silicon nano-particles embedded in a silicon dioxide (SiO2 or glass) matrix, reduces many of the problems associated with bulk silicon, and when doped with erbium, or other rare earth material, can exhibit efficient room temperature rare earth luminescence, because of the high efficiency of the energy transfer process from excited nanocrystals to rare earth ions. Accordingly, the SRSO provides an alternative to thin film electroluminescent material. The silicon nano-particles act as classical sensitizer atoms that absorb incident photons or electrons and then transfer the energy to the rare earth ions, which then fluoresce in the infrared or visible wavelength ranges with several advantages compared to the direct fluorescence of the rare earth. First, the absorption cross-section of the silicon nano-particles is larger than that of the rare earth ions by more than three orders of magnitude. Second, as excitation occurs via an Auger-type interaction or via a Förster transfer process between carriers in the silicon nanoparticles and rare earth ions, incident photons need not be in resonance with one of the narrow absorption bands of the rare earth. Unfortunately, existing approaches to developing such silicon nano-particle materials have only been successful at producing very low concentrations of the rare earth element, which is not sufficient for many practical applications.
Observations have shown that silicon nano-particles formed by such techniques generally have a relatively narrow distribution of photo-luminescent (PL) wavelength or energy despite the broad size distribution, i.e. the observed energies are not as high as expected from the quantum confinement of the nanocrystals. The reduced nano-particle excitation energy affects the efficiency of energy transfer from conducting electrons when these structures are electrically powered, thereby severely limiting the efficiency of light generation from such films.
In general, the manufacture of type IV semiconductor nano-particles doped with a rare earth element is done by ion implantation of silicon ions into a silicon oxide layer, followed by high temperature annealing to grow the silicon nano-particles and to reduce the ion implantation damage. The implantation of silicon ions is followed by an ion implantation of the rare earth ions into the annealed silicon nano-particle oxide layer. The resulting layer is again annealed to reduce the ion implant damage and to optically activate the rare earth ion.
There are several problems with this method:
i) it results in a decreased layer surface uniformity due to the ion implantation;
ii) it requires an expensive ion implantation step;
iii) it fails to achieve a uniform distribution of group IV semiconductor nano-particles and rare earth ions unless many implantation steps are carried out;
iv) it requires a balance between reducing the ion implant damage by thermal annealing while trying to maximize the optically active rare earth; and
v) the thickness of the film is limited because implanted ions do not penetrate deeply into the film for practical implant energies.
To diminish the above drawbacks, plasma enhanced chemical vapor deposition (PECVD) has been utilized to make group IV semiconductor nano-particle layers. The prepared layers are subjected to a rare earth ion implantation step, and a subsequent annealing cycle to form the group IV semiconductor nano-particles and to optically activate the rare earth ions that are doped in the nano-particle region. Unfortunately, the layers prepared with this method are still subjected to an implantation step, which results in poor surface uniformity, non-uniform distribution of rare earth elements, and limited film thickness.
Another deposition method that has been used to obtain a doped group IV semiconductor nano-particle layer consists of co-sputtering the group IV semiconductor and rare earth metal, typically in an oxygen plasma. In this method, the group IV semiconductor and the rare earth metal were placed on a target substrate, which was then placed into a vacuum chamber and exposed to an argon ion beam. The argon ion beam sputtered off the group IV semiconductor and the rare earth metal, both of which were deposited onto a receiving silicon wafer. The newly formed film on the silicon wafer was then annealed to grow the nano-particles and to optically activate the rare earth ions. The doped group IV semiconductor nano-particle layers made through this method have the drawbacks that: i) the layer does not have a very uniform distribution of nano-particles and rare earth ions; ii) the layer suffers from up conversion efficiency losses due to rare earth clustering in the film; and iii) the concentration of rare earth film in the film is limited to little more than 0.1%.
The concentration of the one or more rare earth elements in the semiconductor nano-particle layers is preferably as high as possible, as the level of response of the film to external stimuli, such as optical stimulation for photoluminescence, is proportional to the concentration. One problem encountered, when a high concentration of rare earth element is present within the semiconductor layer, is that when two rare earth metals come into close proximity with one another, a quenching relaxation interaction occurs that reduces the level of optical response observed. The concentration of the rare earth elements within a semiconductor film is thus balanced to be as high as possible to offer the most fluorescence, but low enough to limit the quenching interactions.
Silicon nano-particles formed by such techniques generally have a relatively wide distribution of size, and a similarly wide spatial distribution, i.e. the separation distance between nano-particles, which affects the efficiency of energy transfer from conducting electrons when these structures are electrically powered. The average distance between nano-particles in the direction of electrical conduction must be large enough so that an electron picks up enough energy from the electric field between nano-particles to excite the light emitting object and produce a photon of the correct colour. However, because the spatial distribution is isotropic, the overall density of nano-particles in these films must be fairly low (˜5×1018 cm−2). Unfortunately, with such a low nano-particles density, and with a distribution of nano-particle size and separation, severe limitations are set on the efficiency of light generating capability from such films with embedded nano-particles.
When rare earth ions are introduced in the film, it is desirable to locate the rare earth ions in the vicinity of the nano-particles to facilitate efficient energy transfer from the excited nano-particles to the rare earth ions. However, the ion implantation or in situ deposition techniques incorporate a random distribution of rare earth ions. In particular, the generation of white light requires multiple species of rare earths to be incorporated into the films, since each different species provides a different colour. It is impossible to ensure that the correct rare earth ion is located near the appropriate size of nano-particle so that the energy of the excited nano-particle is matched to the emissive wavelength of the rare earth ion. In other words, it is highly likely that the nearest rare earth ion radiates with too short a wavelength, i.e. it cannot be excited by the nano-particle, or too long a wavelength, i.e. energy is wasted in the excitation process. Even if a high enough concentration of rare earths is used to avoid (at least partially) such mismatches of excited nano-particle energy to the radiative emission wavelength of the rare earth ion, losses still arise from interactions between rare earth ions when they are closely spaced.
In the films in which the nano-particles are formed with significant variations in size and separation distance, excess silicon atoms and dopants, such as rare earth ions, are incorporated uniformly throughout the films, but non-uniformly from a local viewpoint. Therefore, there is a chance that some of the excess silicon atoms may be located far from any nucleation site and may not precipitate into the nano-particles, but instead will remain distributed in the silicon dioxide host matrix. In addition, some rare earth ions may also not be located close enough to the nano-particles. Finally, if significant carbon content must be incorporated into the nano-particles to raise their excitation energy, the carbon atoms need to be located close to the nano-particles. It has been observed in general that without carbon incorporation, silicon nanocrystals with 2 nm diameter should have exciton energy of the order of 2.3 eV from the quantum confinement effect, but it is observed that they only radiate in the range of 1.4-1.8 eV. If impurities, such as excess silicon atoms, rare earth ions, and carbon atoms, remain in the oxide matrix, they could severely impact the physical properties of the oxide, particularly the breakdown field and hence device reliability and lifetime.
An object of the present invention is to overcome the shortcomings of the prior art by providing a multi-layered engineered structure in which wide bandgap semiconductor or dielectric buffer layers are disposed adjacent very thin active luminescent layers designed to emit light at a specific wavelength. The buffer layers provide the exact distance in the direction of electrical conduction so that an electron picks up enough energy from the electric field when passing through the buffer layers to excite luminescent centers in the active layers to produce a photon of the correct color via impact ionization or impact excitation.